This invention relates to a moisture-curing NCO-reactive polyurethane hotmelt adhesive composition, to a process for its production and to its use as an adhesive.
Moisture-curing one-component NCO-reactive polyurethane hotmelt adhesives (also known as hotmelts) are known as a basis for adhesives and sealing compounds. After application to a substrate, they develop their ultimate strength and heat resistance by an after-reaction of the free NCO groups with water from the atmosphere.
Many applications require rapid setting of the applied reactive hotmelts to allow further processing in short cycle times. In addition, it is important in these applications that there is very little, if any, change in volume during setting and final curing so that no mechanical stresses are built up. Neither should any mechanical stresses be built up by variations in temperature. In addition, the hotmelts used for bonding must show high heat resistance and, in the event of prolonged storage in water, should not allow any of the water to creep through.
An improved, i.e. shorter, setting time can be obtained by addition of resins or thermoplastic polymers (EPA 0 232 055, EPA 0 107 097, EPA 0 246 473). However, the large percentages of thermoplastic resins still present after crosslinking adversely affect shear strength at elevated temperature.
The use of polyesters having a large content of aromatic dicarboxylic acid (EP-A 0 248 658) also improves the setting rate, but leads to products of high melt viscosity (which complicates processing) and, in the cured state, to poor flexibility for many applications. The same also applies where rapidly crystallizing aliphatic polyesters are used (DE-A 3 827 224). The marked tendency towards crystallization leads to a measurable change in volume during curing which, in adhesive bonds, causes the layer of adhesive to separate from the substrate. Where the adhesive bonds are stored in water, this in turn leads to the distinct creepage of water beneath the adhesive layer.
The marked tendency towards crystallization where readily crystallizing aliphatic polyesters are used also presupposes that the adhesives used are applied to preheated substrate. If the adhesives used are applied to non-preheated substrate, "skin formation" occurs through spontaneous crystallization at the surface, distinctly reducing adhesion and promoting the above-mentioned separation from the substrate. An adhesive which could be applied to non-preheated substrate would clearly simplify the application process.
Where hotmelts are used in the automotive industry for bonding windows or headlamp lenses, i.e. glass/plastic composites, rapid curing (short cycle times), minimal shrinkage during curing and resistance to the creepage of water are particularly necessary. The imperviousness of the adhesive bond to moisture has to satisfy stringent requirements.
Accordingly, the problem addressed by the present invention was to develop moisture-crosslinking hotmelt adhesives which would cure flexibly with no stresses after a short setting time, could be applied with advantage to non-preheated substrates and, after curing, would give adhesive bonds highly resistant to the creepage of moisture.
It has now been found that, according to the invention, this problem can be solved by a novel moisture-curing, NCO-reactive polyurethane hotmelt adhesive composition